( 145 ) 



that bright or dark spots, visible on one pliotograph, may be wanting 

 in the other. 



Moreover, the general character of the image mnst change as we 

 approach the central line. For in proportion as the indices of refrac- 

 tion depart fi-om unity, slower variations of density suiïice for pro- 

 ducing sensible differences of divergence. And, as a matter of course, 

 in any whirling region slightly inclined density-gradients will take up 

 larger spaces than very steep ones. Besides, Avhen the second slit of 

 the spectroheliograph, haA'ing a given width, is set near to the central 

 absorption line, the wave-complex which it allows to pass, covers a 

 greater variety of refractive indices, than when it is set farther from 

 the central line. In the former case the distribution of the liijht in 

 the solar image must, therefore, be less differentiated. Both circum- 

 stances cooperate in causing the bright-and-dark structure generally 

 to appear coarser and more woolly in proportion as the spectro- 

 heliograph is adjusted for kinds of rays that are more liable to 

 anomalous dispersion. 



From the same point of view it is not surprising that on photo- 

 graphs, taken in H^ or K, light, the calcium flocculi are parti- 

 cularly bright and extensive in spot regions, for in such regions 

 the "tubular" structure of the gaseous mass, by which the strongly 

 curved rays are kept together and conducted, is most developed. 



Hale and Ellermann also mention "dark calcium flocculi" ^), 

 which they describe as special objects, visible in so-called "high-level 

 photographs" and not to be confounded with the general dark back- 

 ground, produced by the absorbing calcium vapour of deeper layers. 

 Dark flocculi often surround the large bright flocculi of spot regions, 

 as is shown e. g. in Fig. 4, Plate V of the cited publication. The 

 explanation given by them is, that we might have here some indications 

 of the cooler K3 calcium vapour, which rises to a considerably greater 

 height than the K, vapour of the bright flocculi. 



In our theory the presence of these darker regions is a direct 

 consequence of the fact, that the particular distribution of the light 

 in the solar image is not produced by local absori)tion and omission, 

 but by irregular ray-curving. The rays are only caused to change their 

 places ; so an excess of light in the bright flocculi must necessarily 

 be counterbalanced by a deficit in the surroundings. 



H and K are by far the broadest bands of the visible solar spec- 

 trum ; even with moderate dispersion the second slit of the spectro- 



^) Publications Yerkes Observatory, 1. c. p. 19. 



10 



I 



Pi'oceeclings Royal Acad. Amsterdam. Vol. VII. 



